Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA, and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major, free, energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually. Additionally or alternatively, CO rich gas streams (syngas) can be produced by gasification of carbonaceous materials, such as coal, petroleum and biomass. Carbonaceous materials can be converted into gas products including CO, CO2, H2 and lesser amounts of CH4 by gasification using a variety of methods, including pyrolysis, tar cracking and char gasification. Syngas can also be produced in a steam reformation process, such as the steam reformation of methane or natural gas. Methane can be converted to hydrogen and carbon monoxide and/or carbon dioxide by methane reformation in the presence of a metal catalyst. For example, steam reformation of methane occurs as follows:CH4+H2O→CO+3H2  (1)CO+H2O→CO2+H2  (2)
This process accounts for a substantial portion of the hydrogen produced in the world today. Attempts to use the hydrogen produced in the above reactions in fuel cell technology have been largely unsuccessful, due to the presence of carbon monoxide, which typically poisons fuel cell catalysts. Other catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al., Archives of Microbiology 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is always associated with co-production of acetate and/or acetic acid. As some of the available carbon is converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to GHG emissions.
Microbial fermentation of CO in the presence of H2 can lead to substantially complete carbon transfer into an alcohol. However, in the absence of sufficient H2, some of the CO is converted into alcohol, while a significant portion is converted to CO2 as shown in the following equations:6CO+3H2O→C2H5OH+4CO2 12H2+4CO2→2C2H5OH+6H2O
The production of CO2 represents inefficiency in overall carbon capture and if released, also has the potential to contribute to Green House Gas emissions.
WO2007/117157, the disclosure of which is incorporated herein by reference, describes a process that produces alcohols, particularly ethanol, by anaerobic fermentation of gases containing carbon monoxide. Acetate produced as a by-product of the fermentation process is converted into hydrogen gas and carbon dioxide gas, either or both of which may be used in the anaerobic fermentation process. WO2008/115080, the disclosure of which is incorporated herein by reference, describes a process for the production of alcohol(s) in multiple fermentation stages. By-products produced as a result of anaerobic fermentation of gas(es) in a first bioreactor can be used to produce products in a second bioreactor. Furthermore, by-products of the second fermentation stage can be recycled to the first bioreactor to produce products.
U.S. Pat. No. 7,078,201 and WO 02/08438 also describe improving fermentation processes for producing ethanol by varying conditions (e.g. pH and redox potential) of the liquid nutrient medium in which the fermentation is performed. As disclosed in those publications, similar processes may be used to produce other alcohols, such as butanol.
Even minor improvements to a fermentation process for producing one or more acids and/or one or more alcohols can have a significant impact on the efficiency, and more particularly, the commercial viability, of such a process.
For example, regardless of the source used to feed the fermentation reaction, problems can occur when there are breaks in the feed supply. More particularly, such interruptions can be detrimental to the efficiency of production by the micro-organisms used in the reaction, and in some cases, can be harmful thereto. For example, where CO gas in an industrial waste gas stream may be used in fermentation reactions to produce acids/alcohols, there may be times when the stream is not produced. During such times, the micro-organisms used in the reaction may go into an inactive, non-productive state or hibernation. When the stream is available again, there may then be a lag before the micro-organisms are fully productive at performing the desired reaction. Accordingly, there would be significant benefit if there were a means to reduce or eliminate this lag time.
As another example, in many industrial processes, scrubber systems or apparatus are used to reduce the concentration of particulates (such as dust) and other components that contaminate exhaust gases. Dry or wet scrubbing systems are known. In a wet scrubbing system, water or other liquids are used to “scrub” the contaminants from the gas stream. A typical wet scrubbing system is seen in steel mills, where water is used to clean flue gases generated at various stages of steel manufacture: for example gases generated by the coking ovens, the blast furnace, the basic oxygen furnace or the electric arc furnace. While scrubbing has the benefit of reducing the level of contaminants within exhaust gases, it by no means eliminates the contaminants altogether. The unwanted substances are simply removed from the gas into a solid or powder form or into the scrubber water or liquid. The water or liquid used in the scrubber system thus becomes a waste stream generated by this industry. The disposal of such waste represents an environmental hazard. The need to clean and dispose of such waste materials also represents a significant cost to the industry.
While conventional industrial scrubbers (such as at steel mills) remove a portion of the contaminants from industrial waste gas streams, it has been accepted in the art that additional scrubbing and/or treatment steps are required to be performed on the gases before they may be used to feed a fermentation reaction due to the perceived harmful effects of such gases on the micro-organisms used in the reaction. See, for example, Datar et al., Fermentation of biomass-generated producer gas to ethanol, 2004, Biotechnology and Bioengineering Vol. 86, pp 587-594. The use of additional scrubbing and/or treatment steps requires additional space in an industrial plant, which can be particularly problematic where the use of fermentation processes is added to an existing plant. Accordingly, there is a need for improved processes in which such additional scrubbing or other treatment steps are not required or are at least kept to minimum.
It is an object of the present invention to provide system(s) and/or method(s) that overcomes or ameliorates at least one disadvantage known in the art and provides the public with new methods for improved and/or increased production of a variety of useful products.